The chemical industry has in recent years made great strides of progress, with development of new processes and improvements of existing processes being steadily under way toward more labor-saving and more efficient operations. Accompanied with this, there is expanding demand for equipment materials having sufficient corrosion resistance for services in such severely corrosive environments as non-oxidizing acids typified by hydrochloric and sulfuric acids. Particularly, the corrosive environments surrounding the equipment materials are recently becoming more and more rigorous with spreading uses of high-temperature, high-concentration non-oxidizing acids. Also in the processing for disposal of waste liquors, waste gases, and solid refuses which involve in environment problems, the process has increased chances in which hydrochloric and sulfuric acids at high temperatures and high concentrations must be treated. From this viewpoint too, equipment materials capable of safely handling such acids are being required. Besides these circumstances, a more recent tendency is that, from the economical view point, more weight is placed on the total cost including maintenance cost rather than the initial investment only under the consideration of the ease of maintenance. The tendency is reflected by the increasing use of high-grade corrosion-resistant materials for the aforesaid environments.
Those corrosion-resistant materials, roughly divided into metallic and non-metallic, are being used in various fields according to their characteristics. The metallic materials in particular are used in heat exchangers where their heat transfer efficiency is valued and in the main portions of other equipment where their toughness in structure is prized. In view of these, there is a strong demand for metallic materials which combine reliable corrosion resistance with economical efficiency.
The metallic materials known in the art for use in such high-temperature, high-concentration non-oxidizing acids include Nb, Ta, Zr, "Hastelloy (trade mark)", and corrosion-resistant titanium alloys. Among them, Nb and Ta are excellent in corrosion resistance but the extremely high prices limit their utilization in industry. On the other hand, Zr and "Hastelloy" have the problem of their corrosion resistance being deteriorated by the presence of C1.sup.-.
Conventional corrosion-resistant titanium alloys, typified by the Ti-Pd alloy, are not adequately resistant to such non-oxidizing acids as hydrochloric and sulfuric acids. Ti-Mo alloys containing up to several ten percent of molybdenum (e.g., refer to "TRANSACTIONS OF THE ASM", Stern et al., Vol. 54, 1961, p. 286.) and Ti-Mo-Ru alloys enhanced in corrosion resistance by the further addition of a small amount of ruthenium of an relatively inexpensive precious metal (refer to Japanese Patent Application Public Disclosure No. 337389/1989.) are excellently resistant to the corrosive attacks of hydrochloric and sulfuric acids. These Ti-Mo-(Ru) alloys (which designates Ti-Mo alloys or Ti-Mo-Ru alloys) have beta-phase which is homogeneous in structure and therefore are easy to work. For the reason, they can be fabricated as equipment materials into diverse shapes. Moreover, the use of Mo, a metal less costly than Nb and Ta, makes these alloys more economical than the other high-grade corrosion-resistant materials.
The Ti-Mo-(Ru) alloys do prove outstandingly corrosion-resistant in a non-oxidizing acid, such as hydrochloric or sulfuric acid, as long as the acid is free from impurities. However, but when even a few ppm of an oxidizing agent is mixed as a foreign matter into these acids, the Ti-Mo-(Ru) alloys would pose a problem; serious deterioration of the corrosion resistance of the alloys due to overpassivation of Mo. Generally, in actual environments, the ingress of oxidants, such as traces of impure ions like Fe.sup.3+ and Cu.sup.2+ or dissolved oxygen in solution, is common. So, the susceptibility to the corrosive action of the oxidants is a fatal disadvantage that has severely restricted the industrial utilization of the Ti-Mo-(Ru) alloys. As stated above, the Ti-Mo-(Ru) alloys are highly resistant to the corrosive attacks of non-oxidizing acids, exhibit good workability, and provide good economy. Nevertheless, they have the fatal disadvantage as an industrial material of their corrosion resistance being seriously affected by the presence of a trace of an oxidant.